The field of molecular electronics has developed rapidly over the last 15 years with the discovery of organic conductive and semiconductive compounds. Over this time many compounds have been found that display semiconductive or electro-optical properties. Semiconductive organic compounds are currently being developed for applications such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), sensors and photovoltaic elements. A field effect transistor is a three-electrode element in which the conductivity of a thin conducting channel between two electrodes (known as “source” and “drain”) is controlled by means of a third electrode (known as “gate”), which is separated from the conducting channel by a thin insulating layer. The most important characteristic properties of a field effect transistor are the mobility of the charge carriers, which decisively determine the switching speed of the transistor, and the ratio between the currents in the switched and unswitched state, known as the “on/off ratio”. Another important property of a field effect transistor is the inception voltage at which a measurable current starts to flow between source and drain. This voltage is also known as the threshold voltage. Low threshold voltages are generally desirable. In order to reduce this threshold voltage, developers try to make the insulating layer between the gate and the conducting channel as thin as possible.
The thinnest possible insulating layers can be produced from so-called “self-assembled monolayers” (SAMs). Examples of suitable molecules from which SAM layers can be produced are long linear alkanes having more than 10 carbon atoms, which can be anchored to the supporting material of the transistor by means of suitable functional groups in the molecule. U.S. Pat. No. 6,433,359 B1 describes the use of linear, branched or cycloaliphatic hydrocarbons having polar groups, such as e.g. chlorosilyl, carboxyl, hydroxyl, amino, amido and thiol groups, for the production of field effect transistors containing SAM layers between the insulating layer and the semiconductor layer to improve the on/off ratio, threshold voltage (i.e. the voltage at the gate at which a measurable current starts to flow between source and drain), and the mobility of the charge carriers. The use of SAM layers containing thiol groups with a similar objective is described in U.S. Pat. No. 6,335,539 B1.
However, the best properties have been found in field effect transistors in which layers made from SAM molecules are used as the sole insulating layer. For example, field effect transistors that display a current between source and drain at a threshold voltage of 1–2 V have been produced with an approximately 2 nm thin SAM insulating layer consisting of alkyl trichlorosilanes (see, e.g., J. Collet et al., Appl. Phys. Lett. 1998, Vol. 73, No. 18, 2681–2683 and Appl. Phys. Lett. 2000, Vol. 76, No. 14, 1941–1943).
The disadvantage of alkyl-containing SAM layers, however, is that they are difficult to structure and because of the low surface voltage of approximately 20 to 30 mN/m they cause difficulties when it comes to applying additional layers to these layers (e.g., by the wet process). By means of functionalisations, for example of one end of the SAM molecule, with carboxyl groups for example, the surface voltage was able to be increased to around 50 mN/m (see, e.g., Appl. Phys. Lett. 2000, Vol. 76, No. 14 p. 1941–1943).
Linking the alkyl(ene) chains in the SAM molecule to semiconductive, conjugated oligomers, such as oligothiophenes for example, also made it possible to achieve an improvement in the surface voltage between the insulating layer and the semiconductive layer on the one hand and an ordered construction of two layers, the dielectric and the semiconductive layer, in one step on the other. However, although such SAM molecules with oligothiophenes at one end and thiol groups at the other end of an alkylene group had already been described, such as 12-(2,2′:5′,5″,2′″-quaterthien-5-yl)dodecanethiol (see, e.g., Colloids Surf. A, 198–200 (2002) 577–591) or 11-(2,2′:5′,2″-terthien-5-yl) undec-1-ylthiol (see, e.g., Bäuerle et al., J. Phys. Chem. B 1997, Vol. 101, No. 31, 5951–5962), it is known that compounds containing thiol groups have the disadvantage in the production of SAM layers that they can only be anchored to gold surfaces.
Berlin and Zotti et al. (J. Am. Chem. Soc. 1998, B.120, p. 13453–13460) describe carboxyalkyl-substituted dithiophenes and terthiophenes which are suitable for forming monolayers on indium-tin-oxide layers (ITO). The disadvantage of these compounds, however, lies in the fact that dithiophenes and terthiophenes display no semiconductive properties and are typically only suitable as intermediates for other reactions. Thus dithiophenes or terthiophenes have been polymerised by oxidation after being applied as a layer, for example, but doped conductive polymers were obtained in this way that display no semiconductor effect. The molecules described therein are thus unsuitable as semiconductors both before and after polymerisation.
Other SAM compounds functionalised with a thiophene, pyrrole or other aromatic rings at the end of the molecule, such as e.g. 11-(3-thienyl)undecyl trichlorosilane (Mat. Res. Soc. Symp. Proc. 2002, B.708, p. 305–309) or similar compounds (US-A 2003/0,099,845) have been described. However, these compounds likewise display no semiconductive properties. Following oxidative polymerisation they form electrically conductive polymer layers anchored to the substrate, such as e.g. polypyrrole, polythiophene, polyacetylene and polydiacetylene layers, which are in the oxidised, electrically conductive form and not in the neutral, semiconductive form, however.
There is therefore still a need for SAM molecules that are suitable both (i) for the formation of thin insulating layers on suitable substrates and, (ii) for the formation of semiconductive layers.